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In a deadlock situation, none of the sessions can continue to execute until one of those sessions releases its locks, so allowing the other session(s) access to the locked resource.Multiple processes persistently blocking each other, in an irresolvable state, will eventually result in a halt to processing inside the database engine.

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SQL Server 2008 provides all of these facilities, plus the system_health Extended Events Session.

To allow you to work through each section, and generate the same deadlock graphs that are presented and described in the text, the resource materials for this article include example code to generate a deadlock in SQL Server.

When Trace Flag 1204 is enabled for all sessions on a SQL Server instance, any deadlock detected by the deadlock monitor will cause a deadlock graph to be written to the SQL Server error log.

In SQL Server 2000, this Trace Flag is the only means by which to capture a deadlock graph, which makes troubleshooting deadlocking in SQL Server 2000 quite challenging, though still possible.

A common misconception is that DBAs need to intervene to “kill” one of the processes involved in a deadlock.

In fact, SQL Server is designed to detect and resolve deadlocks automatically, through the use the Lock Monitor, a background process that is initiated when the SQL Server instance starts, and that constantly monitors the system for deadlocked sessions.Trace Flags in SQL Server enable alternate “code paths” at key points inside the database engine, allowing additional code to execute when necessary.If you are seeing queries failing with deadlock errors on a SQL Server instance, Trace Flags can be enabled for a single session or for all of the sessions on that instance.It captures and presents the information in a manner that makes it much easier to identify the deadlock victim, as well as the resources and processes involved in the deadlock (covered in detail in the section).Trace Flag 1204 is still available, for backwards compatibility reasons, but when using Trace Flags to capture deadlock graphs in SQL Server 2005 or later, you should always use Trace Flag 1222 in preference to Trace Flag 1204.However, when deadlocks are reported, the DBA must investigate their cause immediately.

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